I'd say things like REITs, as others have mentioned above. I personally prefer syndications as opposed to REITs because of REITs lower Sharpe Ratio in general, and in syndications where you get to see the underwriting in every single deal, you can get a lot deeper in how the operator thinks about the opportunity.
A lot of folks talk about rental real estate as 'passive' -- I've had some in the B+/A- class, but it was far from 'passive'. I mean, most months would go along fine, but when there's like a garage door breaking, or some plumbing issue, or a gas leak, or vacancy, it's always an extremely stressful scenario.
So while I'm not actively managing the property myself, I'd say it isn't mentally passive at all. Nor is it passive when you have to make decisions about certain things. Even if you have a property manager, they'll defer to you as to whether you want to use this paint or that paint during a turnover, or what line items you'd like to be ripped off on. I made more in my passive syndications than I did picking out deals myself, and it was a lot less stressful.
Of course, that's just my experience. Your mileage may vary.